The present invention pertains to systems and methods for activating contractile actuators based on ion migration, and, more particularly, to methods for achieved increased activation rate without significant degradation of actuator lifetimes.
Polypyrrole, polyaniline, polyacetylene and other conducting polymers undergo volumetric changes that may be exploited to perform work, as described by R. H. Baughman, Synthetic Metals 1996, 78(3), 339-353 and A. Mazzoldi, A. Della Santa, D. De Rossi, Polymer Sensors and Actuators, Springer Verlag: Heidelberg, 1999, each of which are hereby incorporated by reference, in their entirety. Polymer expansion is generally the result of ionic and molecular influxes that occur as oxidation state is altered either chemically or electrochemically, as described by R. H. Baughman, R. L. Shacklette, R. L. Elsenbaumer, Topics in Molecular Organization and Engineering, Vol. 7: Molecular Electronics, Kluwer: Dordrecht, 1991, p 267; Q. Pei, O. Inganas, Journal of Physical Chemistry, 1992, 96(25), 10507-10514; T. E. Herod, J. B. Schlenoff, Chemistry of Materials 1993, 5, 951-955; M. Kaneko, M. Fukui, W. Takashima, K. Kaneto, Synthetic Metals 1997, 84, 795-796; and T. F. Otero, Handbook of Organic and Conductive Molecules and Polymers, John Wiley & Sons: Chichester, 1997; Vol. 4, pp 517-594, each of which are hereby incorporated by reference, in their entirety. Electrochemical experiments show that strain is proportional to the magnitude of charge transfer and, equivalently, that strain rate is proportional to current.
Strains for non-gels are commonly in the range of one to three percent at applied stresses of up to 5 MPa, but can exceed 10% in chemically doped material. Mammalian skeletal muscle, by comparison, deforms by 20% in vivo with stresses of 0.35 MPa. The integral of stress with respect to strain over a cycle is the work density. Conducting polymers currently match muscle in this figure of merit, but are three orders of magnitude slower and, as a consequence, are much less powerful for a given volume or mass. Strain rates reported to date reach 0.1%·s−1 with associated power to mass ratios of <1 W·kg−1. Faster responses are likely achieved in a number of bilayer actuators, as described by K. Kaneto, M. Kaneko, Y. Min, A. G. MacDiarmid, Synthetic Metals 1995, 71, 2211-2212, which is hereby incorporated by reference, in its entirety, but their intrinsic characteristics are not reported, as described by J. D. Madden, S. R. Lafontaine, I. Hunter, I. W. Proceedings—Micro Machine and Human Science 95; 1995, which is hereby incorporated by reference, in its entirety. Bilayers are laminates of conducting polymer and other thin films, in which the relative expansion or contraction of the polymer with respect to the other layers leads to a bending of the structure, with small material strains being amplified to create large deflections. Using the bilayer deflection data reported by E. Smela, O. Inganas, I. Lundstrom, Science 1995, 268, 1735-1738, which is hereby incorporated by reference, in its entirety, the tensile modulus polypyrrole grown using their methods, and the standard relationship between deflection and strain, we calculate that the 400 nm thick polypyrrole/gold bilayers Smela et. al. employ achieve strain rates of 1% s−1 (still roughly 100 times slower than mammalian skeletal muscle).